Static protected detonator



May 9, 1967 P. H. MILLER STATIC PROTECTED DETONATOR Filed May 5, 1964 INVENTOR. Pou H. Miller 3,318,243 STA'HC PROTECTED DETGNATfiR Paul H. Miiler, Tamagua, Pa, assignor to Atias Chemical industries, lino, Wilmington, lDelL, a corporation or" Deiaware Filed May 5, 1964, Ser. No. 364,916 8 Claims. (Cl. 1%2-28) The present invention relates to an electric explosion initiator having improved protection from static electricity and more particularly to an initiator sealing plug which provides improved protection from static electricity.

It has long been recognized that extraneous electrical energy, particularly that in the form of static electricity, in blasting circuits can result in premature ignition. Ignition compositions are generally highly heat sensitive and exposure of a bridge wire to a static discharge may ignite the adjacent ignition composition.

Although the danger from accidental firing of detonators by extraneous electricity is present in almost all blasting operations, premature ignition is most possible when the blasting operation either is carried out under conditions conducive to high levels of either atmospheric static electricity, for example, in electrical, dust or snow storms, or is carried out under conditions conducive to the generation of static charges, for example, operations wherein a particulate blasting agent, such as sensitized ammonium nitrate is blowloaded, e.g., loaded into a bore hole by a stream of compressed air.

A number of solutions to the problem of static susceptibility of electric initiators are known, and most of them have been effective at least to some degree. However, none have been found to be entirely satisfactory when cost, ease of manufacture, effectiveness, and storage life are all considered.

Generally the prior art solutions to the static electricity problem require an additional component to be added either in or on the initiator. This addition usually in creases the size of the initiator or decreases the amount of explosive that may be included in the initiator. It is preferred that initiators be compact in size in order that the initiator may be placed into small initiator Wells, punched or slitted holes in explosive charges and yet insure the propagation of a detonation wave from the initiator to the explosive charge. The present invention provides protection from static electricity without an increase in size or a decrease in the amount of explosive which may be placed within the initiator.

In accord with the present invention an electric explosion initiator is provided which has a sealing plug that provides an electrical by-pass for static electricity. The electric explosion initiator comprises a shell member containing an explosive charge positioned therein, an igniting assembly positioned in firing position with respect to the explosive charge, and a sealing plug. In accord with the present invention a sealing plug is provided which is comprised of a matrix of flexible binding material of high electrical resistance which contains a dispersion of a secondary electron emitter and a non-linear resistance material. A pair of leg wires is positioned to pass through the plug and be both electrically connected to the igniting assembly and in electrical contact with the plug member. The leg wires are bared along a portion of their length and the bared portion is in physical contact with the plug.

Suitable flexible binding materials are those having an electrical resistivity greater than about 10 ohms-centimeters. Suitable binding materials include natural rubber, synthetic-natural rubber, synthetic rubber and flexible plastic polymers. For example, some materials which are suited to use in the present invention are natural gum nited States Patent rubber, isoprene synthetic rubber, silicone rubber and polysulfide rubber.

Secondary electron emitters are materials that emit electrons when bombarded with electrons from a primary source. Examples of secondary electron emitters suitable for use in the present invention are: potassium iodide, antimony ceside, quartz, glass, and metal oxides, such as, oxides of aluminum, barium, beryllium, cadmium, calcium, cesium, chromium, copper, iron, lead, magnesium, tin, strontium, and zinc. Preferably the secondary electron emitters are of a size that will pass a 325 U.S.S. screen.

A non-linear resistance material is a material that exhibits a non-linear resistance to the flow of electric current, that is, the material does not exhibit a resistance in accord with that predictable by Ohms law. To illustrate, Ohms law, which applies to all linear resistance materials may be written:

where I=current in amperes Ezpotential in volts R=resistance in ohms n=dimensionless and equal to 1 In contrast, for a non-linear resistance material, Ohms law is modified into the form:

where 1=same as above Ezsame as above =reciprocal of resistance when one volt of potential is applied n=dimensionless, generally having a value of between 2 and 14, the relationship being that the greater the nonlinearity, the higher the value assumed by 11.

Examples of suitable non-linear resistance materials are electrical grade silicon carbide and lead sulfide. Electrical grade silicon carbide products sold by Norton Company, Refractories Division under the designations of E-179 and E-254 Crystolon are particularly useful in fabricatin-g sealing plugs adapted to use in the present invention. Preferably the non-linear resistance materials are of a size that -will pass a mesh U.S.S. screen and be retained on a 400 mesh U.S.S. screen.

The following proportions, based on percent by weight, of the protective plug components have been found to be useful in carrying out the present invention: from about 40 to about 55% flexible matrix material, from about 40 to 60% non-linear resistance material and from about 0.5 to about 15% of a secondary electron emitter.

The attached figure is a sectional elevation view of an initiator containing a protective plug in accord with the present invention. As shown in the figure, shell ll, suitably of a metal such as copper, brass or aluminum, contains a base charge in one end thereof. Base charge 13 is suitably a detonatabie material for example, pentaerythritol tetranitrate or tetryl. A primer charge 15 is positioned adiacent and in detonating relation to the base charge 113. Primer charge 15 is suitably comprised of mannitol hexanitrate or diazodinitrophenol. Adjacent and in igniting relation to the primer charge 15 is an ignition assembly comprised of a match assembly which includes leg wires 17 and 19 electrically connected to match tabs 21 and 23, which are separated along their length by insulation 25 and electrically connected at one end by bridge wire 27. Surrounding bridge wire 27 and in intimate contact therewith is match composition 2?. Match composition 29 may suitably be applied around the bridge wire by suspending the match composition in a lacquer, such as nitrocellulose, which acts as a binder and leaves a flammable residue on drying, and dipping the bridge wire end of the match assembly in the suspension. Upon removing the match assembly from the suspension a layer of the suspension clings to and covers the bride wire. On drying a match head is formed. After the first layer, subsequent layers may be applied. Suitable materials for the match ignition composition include, for example, cuprous acetylide, diazodinitrophenol, lead mononitroresorcinate, mixtures of lead mononitroresorcinate and potassium chlorate, and silver or lead azides. An insulating cylinder suitably of paper 31 is positioned between the inside wall of shell 11 and the match assembly in order that physical or electrical contact between the shell wall and the match assembly is prevented. Leg wires 17 and 19 pass through protective plug 33 which in turn is in close physical contact with the inside Walls of shell 11 sealing the open end of shell 11. In order to provide good electrical contact between leg wires 17 and 19 and plug 33, the leg wires are bared along portions 35 and 37. Plug 33 is comprised of electrical grade silicon carbide, a mixture of silicon carbide and alumina which has been fused at a high temperature, and a secondary electron emitter dispersed in a matrix of silicone rubber. Plug 33 may be fabricated by molding a composition of 100 parts by weight of silicone rubber, 105 parts by weight of E179 Crystolon (an electrical grade of silicon carbide), of 240 mesh U.S.S. screen, 2.5 parts by weight of magnesium oxide of 325 mesh U.S.S. screen, and 1.0 part of Thermolite 12 Curing Agent (an organotin curing compound produced by Metal & Thermit Corp.). The plug may be molded about the bared section of the leg wires or, if desired the plug may be molded separately and the leg wires inserted through holes which may have been molded, drilled or pierced in the plug, in order to receive and tightly hold the bared leg wires. Suitably plug 33 contains 50.4% by weight of the non-linear resistance material. Plug 33 provides a path of high electrical resistance between shell 11 and leg wires 17 and 19 when exposed to normally low voltages, say from to 800 volts, but a low resistance to high voltages comparable to those required to initiate an electric initiator by a static electrical charge. The length of bared portions and 37 of leg wires 17 and 19 is suflicient to insure good electrical contact and consequently a good electrical discharge path. A bared length of about A" has generally been found to be satisfactory when dealing with standard size electrical initiators.

In order that an essentially constant electric field is maintained in the protective plug, it is preferred that bared portions 35 and 37 of leg wires 17 and 19 be spaced along their lengths equidistant from shell 11. This may be accomplished by positioning the wires symmetrically on opposite sides of the center axis of the plug. The wires may be so positioned by initially aligning the leg wires and molding a plug around the aligned wires. An alternate method is to mold aligned holes through solid plugs and insert the leg wires through these holes. The distance that the leg wires are separated and the distance between the leg wires and shell wall may be varied in accord with the desired conductivity of the plug in the electrical field in which it is exposed. Generally a space of from about 0.044 to about 0.074 inch between the leg wires and the shell wall has been found eminently satisfactory.

An initiator of the present invention containing a protective plug was tested for voltage breakdown by shunting the extending ends of the leg wires and applying a source of DC. voltage between the shunted leg wires and the shell wall of the initiator. The voltage was raised from zero to 1000 volts at a rate of about 20 volts per millisecond. This test applied a voltage across the protective plug. The results of the test indicated that the plug changed from an insulator to a conductor between 4 920 and 1000 volts DC. The test was repeated on six additional initiators and the results were consistent with the initial test.

An initiator of the present invention containing a protective plug was tested for susceptibility to stray current.

In this test a 425 volt A.C. source (600 volts peak-topeak) was applied between the shunted leg wires and the shell wall for a 30 second interval. About 7 initiators were tested in this manner. There was no indication of electrical conduction by the plug in any of the tests.

An initiator of the present invention containing a pro tective plug was tested to determine the maximum electrostatic energy that it could withstand. In this test a potential of 10 kv. D.C. was allowed to discharge from the shunt to the shell. The initiator and 24 additional initiators of identical construction withstood a 1.0 joule shunt-to-shell discharge at 10 kv. without detonation.

What is claimed is:

1. An electric explosion initiator of the type having a shell member containing an explosive charge positioned in said shell member, an igniting assembly positioned in firing relation to said charge, a sealing plug in physical contact with said shell member, and a pair of leg wires passing through said plug in electrical contact therewith and electrically connecting said igniting assembly, wherein the composition of the sealing plug comprises:

(A) A martix of flexible binding material of high electrical resistance having therein;

(1) a dispersion of secondary electron emitter material and a non-linear resistance material;

(2) said secondary electron emitter material having a particle size which passes through a 325 mesh U.S.S. screen; and

(3) said non-linear resistance material having a particle size that will pass a 60 mesh U.S.S. screen and be retained on a 400 mesh U.S.S.

screen.

2. An electric explosion initiator of claim 1 wherein the non-linear resistance material of the sealing plug is a member selected from the group consisting of electrical grade silicon carbide and lead sulfide.

3. An electric explosion initiator of claim 1 wherein the secondary electron emitter material of the sealing plug is a member selected from the group consisting of potassium iodide; antimony ceside; quartz; glass; and oxides of aluminum, barium, beryllium, cadmium, calcium, cesium, chromium, copper, iron, lead, magnesium, tin, strontium, and zinc.

4. A protective sealing plug for use in an electric explosion initiator which comprises:

(A) about 40% to about 55% by weight of flexible matrix material;

(B) about 40 to 60% by weight of non-linear resistance material, said non-linear resistance material having a particle size that will pass a 60 mesh U.S.S. screen and be retained on a 400 mesh U.S.S. screen; and

(C) about 0.5% to about 15% by weight of secondary electron emitter material, said secondary electron emitter material having a particle size which passes through a 325 mesh U.S.S. screen.

5. A protective sealing plug of claim 4 wherein the flexible matrix material is selected from the group consisting of natural rubber, synthetic rubber and flexible plastic polymers.

6. A protective sealing plug of claim 4 wherein the secondary electron emitter material is selected from the group consisting of potassium iodide; antimony ceside; quartz; glass; and oxides of aluminum, barium, beryllium, cadmium, calcium, cesium, chromium, copper, iron, lead, magnesium, tin, strontium, and zinc.

7. A protective sealing plug of claim 4 wherein the non-linear resistance material is selected from the group consisting of electrical grade silicon carbide and lead sulfide.

8. An electric explosion initiator which comprises:

(A) a shell member;

(B) an explosive charge positioned in said shell member;

(C) an igniting assembly positioned in firing relation to said explosive charge;

(D) a sealing plug in physical contact With said shell member, said sealing plug formed of:

(1) about 40% to about 55% by Weight of flexible matrix material;

(2) about 40% to 60% by Weight of non-linear resistance material having a particle size that will pass a 60 mesh U.S.S. screen and be retained on a 400 mesh U.S.S. screen; and

(3) about 0.5% to about 15% by Weight of secondary electron emitter material having a par- 1 ticle size which passes through a 325 mesh U.S.S. screen; and (E) a pair of leg Wires passing through said sealing References Cited by the Examiner UNITED STATES PATENTS 2,086,548 7/1937 Handforth 10228 2,408,124 9/1946 Rolfes 102-28 2,589,157 3/1952 Stalhane 33 82l 2,707,223 4/1955 Hollmann 33820 X 3,128,703 4/1964 Bryan 102-28 3,132,586 5/1964 Scharwachter 10228 3,162,831 12/1964 Heath 338-21 BENJAMIN A. BORCHELT, Primary Examiner. R. V. LOTTMANN, V. R. PENDEGRASS,

Assistant Examiners. 

1. AN ELECTRIC EXPLOSION INITIATOR OF THE TYPE HAVING A SHELL MEMBER CONTAINING AN EXPLOSIVE CHARGE POSITIONED IN SAID SHELL MEMBER, AN IGNITING ASSEMBLY POSITIONED IN FIRING RELATION TO SAID CHARGE, A SEALING PLUG IN PHYSICAL CONTACT WITH SAID SHELL MEMBER, AND A PAIR OF LEG WIRES PASSING THROUGH SAID PLUG IN ELECTRICAL CONTACT THEREWITH AND ELECTRICALLY CONNECTING SAID IGNITING ASSEMBLY, WHEREIN THE COMPOSITION OF THE SEALING PLUG COMPRISES: (A) A MATRIX OF FLEXIBLE BINDING MATERIAL OF HIGH ELECTRICAL RESISTANCE HAVING THEREIN; (1) A DISPERSION OF SECONDARY ELECTRON EMITTER MATERIAL AND A NON-LINEAR RESISTANCE MATERIAL; (2) SAID SECONDARY ELECTRON EMITTER MATERIAL HAVING A PARTICLE SIZE WHICH PASSES THROUGH A 325 MESH U.S.S. SCREEN; AND (3) SAID NON-LINEAR RESISTANCE MATERIAL HAVING A PARTICLE SIZE THAT WILL PASS A 60 MESH U.S.S. SCREEN AND BE RETAINED ON A 400 MESH U.S.S. SCREEN. 